Today all industries must deal with volatile crude oil prices and additional investment are required to meet more stringent environmental regulations. Therefore, optimization of energy usage through more efficient heat exchanger network retrofit and heat integration with the cost reduction are playing an extensive attraction. The grass-root and retrofit designs of heat exchanger network for process industries are addressed in this paper. A strategy is proposed as the MILP formulation based on the special transshipment structure concept. This methodology can generate networks were utility cost, heat exchanger areas and selection of matches are optimized simultaneously. In addition, the simplicity in model assumption, non-isothermal mixing, comes with handling constraints such as stream splitting and allowed/forbidden matches which bring the model structure more convenient to use. The application examples presented here show that the automatic rigorous MILP model can be successfully applied to the complex hot and cold process streams which provides a profitable network for both grass-root design and retrofit, 24,06% total cost saving in the case study of crude distillation unit. This approach also gives a clear advantage over Hypertargets method (Briones, 1999) with more saving about 118 k$/yr of total cost. Finally, in special scenario, relocation topology can be used for further reduction in total cost. The MILP also gives the highest annual cost saving for retrofit HEN by comparing with the approaches of Ciric et al. (1989) and Kin-Lung et al. (2000).